Structural, Transport and magnetic Properties of

International Journal of Scientific & Engineering Research, Volume 5, Issue 3, March-2014
ISSN 2229-5518
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Structural, Transport and magnetic Properties of
Ca2Zn2 Fe12O22Hexagonal Y-Ferrites
C. P. Chaudhari
Department of Physics, Govt. Science College, Gadchiroli, India
Email: [email protected]
AbstractCalcium hexagonal ferrites doped Ca 2 Zn 2 Fe 12 O 22 was prepared
by solid state diffusion technique. The structural properties of
this polycrystalline sample were studied using powder X-ray
diffraction method. The XRD pattern showed a single phase of
the Y type hexagonal ferrite with lattice parameters a=5.93 Å
and c=41.32 Å. The two electrode method was used to study the
DC Resistivity of the ferrite between the temperatures from 300
K to 800 K. The resistivity result showed the behaviour of
samples as semiconductor. Variation in molar susceptibility of
the hexagonal ferrite between temperatures 300 K to 800 K was
studied by Gouy’s balance method. This showed the
ferrimagnetic behaviour of the sample at room temperature. Its
Curie Temperature was found to be 400 K and Curie molar
constant was 52.5
Keywords: Hexagonal ferrites,
resistivity, Ca- Y ferrite.
Solid state
diffusion,
DC
therefore play important role in determining the magnetic
properties of the hexagonal ferrites [4]. The Y-type hexagonal
ferrites have a crystalline structure built up as a superposition
of S and T blocks. The unit cell is composed of the sequence
STSTST including three formula units. Gorter [5] made the
first attempt to determine the position of the magnetic ions
and orientation of the spins in the crystal lattice by
considering exchange interactions. The metallic cations are
distributed among six sublattices as two tetrahedral and four
octahedral sublattices. The distribution of Zn2+and Fe2+
between th blocks is described by parameter γ: S block
contains γ of Zn2+ ions and (1-γ) of Fe2+ ions, while the
opposite holds for the T block [6]. It is worth noting that
inside the T block, three octahedral ions, belonging to 6cVI
and 3bVI sublattices, lies on a vertical 3-fold axis, the central
3b VI ion sharing two faces of its coordination figure with the
adjacent 6c VI ions. Such a configuration is responsible for a
higher potential energy of the structure due to a stronger
electrostatic repulsion between the cations. Such sites are
likely to be preferred by low charge ions. As a consequence
non-magnetic Me2+ ions with a marked preference for the
octahedral coordination may cause drastic changes in the
magnetic configuration with respect to the usual Gorter [5]
scheme; indeed the occupation of either 6cVI and 3bVI by nonmagnetic ions leads to the cancellation of the
antiferromagnetic bVI- c IV * interaction which is the strongest
one in the Y-structure. The substitution of Ba2+ ion by other
divalent cations (i.e. Ca2+ ions in the present case) does not
affect the site distribution [7]. The hexagonal ferrites are
largely used in electronic communication, microwave devices
[8], magneto-optic recording media [9], transformer cores and
high quality filters due to their versatile magnetic properties
[10-14]. Y-type ferrites attracted large interest in recent years
due to the observation of magneto-electric effects.
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I. INTRODUCTION
The basic crystallographic and magnetic properties of the
main hexagonal ferrites have been reviewed by J. Smit and
H.P.J. Wijn [1]. Hexagonal ferrites are classified on the basis
of their chemical composition and crystal structure into six
types: M, W, X, Y, Z, and U. It is possible to synthesise a
ferrite of specific properties by controlling the chemical
composition and nature of substituting element [2]. The
hexagonal ferrites can be prepared by solid state diffusion
technique, firing stoichiometric amounts of oxides MO
(M=Ba, Sr, Pb, Ca etc), Fe 2 O 3 and MeO (Me=Zn, Mg, Mn,
Co etc, the divalent metal ions). This method is most
convenient and simple for the preparation of polycrystalline
ferrites of high purity. In this method the formation of
compounds depends on many factors such as particle size of
reactants, sintering temperature and atmosphere [3]. The
crystalline and Magnetic structures of the different types of
hexagonal ferrites are remarkably complex and can be
considered as a superposition of fundamental structural
blocks:S, R and T blocks.The S block having spinel structure
is a result of cubic close packing of two oxygen layers (O 4 O 4 ) with composition Fe 6 O 8 . The R and T blocks result from
an hexagonal close packing of oxygen layers: R block from
three oxygen layers (O 4 -BaO 3 -O 4 ) and the T block from four
oxygen layers (O 4 -BaO 3 -BaO 3 -O 4 ) respectively, with part of
the oxygen substituted by Ba, Sr, Ca etc. The metallic cations
Me are distributed among several lattice sites different from
the crystallographic and the magnetic point of view and
II. EXPERIMENTAL PROCEDURE
A. Powder Preparation
The single phase polycrystalline sample of hexagonal
ferrite with formula Ca 2 Zn 2 Fe 12 O 22 were synthesized by solid
state diffusion technique. Analar grade powders of CaO, ZnO,
and Fe 2 O 3 are first dried separately at 150 oC for about an
hour to remove the traces of moisture and are mixed
intimately in 1:1:3 proportions. The powdered mixture of the
oxides is ground using A.R. grade acetone in an agate morter
for about 4 hours. To obtain the particle size less than 50
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International Journal of Scientific & Engineering Research, Volume 5, Issue 3, March-2014
ISSN 2229-5518
micron, the sieve with 50 micron filter size was used to filter
the grounded mixture. The larger size particles are again
grounded until their size becomes less than 50 micron. It is
known that finer the particle, swifter is the formation [15].
After grinding the mixture, it was dried in air. The powder
was then mixed separately with 5% polyvinyl acetate (PVA)
in A.R. grade acetone as binder (16). About 2 ml of binder per
gram of the powder is found to give well-moulded pellets
(17). The sample was then pressed in a stainless steel die
using hydraulic press under uniaxial pressure of 5 tonne/sq.
inch for 5 min to form the pellets of 15 mm in diameter and
about 4-7 mm in thickness using. The pellets thus formed are
heated at 300oC for about 4 hours to remove the binder. The
samples were then heated at 11000C for 120 hours so that
ceramic product is obtained. Finally the temperature of the
furnace is cooled down at the rate of 50oC per hour up to
500oC and then cooled down in natural way. The product
masses thus formed are taken out and finely grounded for
about half an hour in the agate mortar. After drying in air the
powder was ready for experimental investigations [18].
B. Pellet preparation
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properties like coercivity, retentivity and saturation
magnetization were found using hysteresis loop tracer HLTIII, made by Scientific Equipments, Roorkee.
III. RESULT AND DISCUSSION
A. XRD analysis
Fig; 1 X-ray diffraction pattern of Ca 2 Zn 2 Fe 12 O 22
The XRD pattern is shown in Fig. 1. This pattern is
matched with the standard pattern which is best fitted with the
hexagonal ferrite and no extra lines were detected. This
showed a single phase of the Y type hexagonal ferrite with
lattice parameters a=5.93 Å and c=41.32 Å. These are in
agreement with findings of Kohn and Eckart [22], that “all the
rhombohedral and hexagonal ferrites are having the same
basal plane (5.90 Å) but differing by the ‘c’ axis stacking of
specific blocks”. The reported value of ‘c’ in Ba-Y ferrite is
43.56 Å [23] which is less as compared to observed ‘c’ in CaY ferrite in the present case. The reason can be given as
follows. The length ‘c’ depends on the distance between two
layers of oxygen, which is the largest ion of all the ions
constituting Ca-Y ferrite. The substituting metal ions occupy
the interstitial positions between the two oxygen layers and
the size of metal ions as well as of Ca2+ ions are so small that
it cannot affect the distance between two oxygen layers. But
in Ba-Y ferrite barium ions are somewhat larger than oxygen
ions which result in increase in distance between the two
adjacent oxygen layers from 2.32 Å to 2.40 Å [24]. These
parameters lead to the rhombohedral rather than hexagonal
. Since rhombohedral
crystal structure with a space group
cell may be referred to a hexagonal cell, the compound is
indexed with reference to hexagonal axes.
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For electrical conductivity measurements again the pellet
was prepared from powder sample. The end faces of the pellet
were gently ground under distilled water. The moisture of the
pellet was removed by oven heating. This is necessary
because surface layer formed during firing may present
difficulties. This layer can have higher resistivity than the
bulk, owing to re-oxidation, particularly at the surface of the
pellet during cooling, or lower resistivity than the bulk as
found with mixed Ni-Zn ferrites of spinel structure [19]. A
Thin layer of silver paste is applied on the both the flat
surfaces of the pellet to provide good electrical contacts [20].
The contacts are dried by heating the pellet slowly at 300oC
for 5 hours.
C. Characterization
The crystalline phase determination of the sample was
carried out on Philips Holand XRD unit (PW 1710) operating
at 42.5 KV and 18.00 mA with filtered Cu K α radiation with
wavelength 1.5405 Å. The 2θ Bragg angles were scanned over
a range of 20o to 100o with a step of 0.02 o for 1 second. XRD
pattern was subjected to Debye Scherrer technique for
checking and identifying the material. The electrical
conductivity of Zn, Ca-Y ferrite was measured from 300 K to
800 K using two electrodes. A small variation in temperature
2oC per minute is maintained throughout the experiment.
Magnetic susceptibility of the samples was measured from
300 K to 800 K by Gouy’s method [21]. Other magnetic
B. Transport properties
Fig. 2 shows the graph of log of electrical resistivity
versus temperature
. The dc electrical resistivity (ρ)
measurements were carried out by two-electrode method. The
graph of log ρ versus inverse absolute temperature is shown in
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International Journal of Scientific & Engineering Research, Volume 5, Issue 3, March-2014
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Figure 4. It is observed that the slope of the log ρ versus
inverse absolute temperature curve changes at the Curie point
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2) Hysteresis loop properties: Hysteresis loop is shown
in Figure 4. From loop saturation magnetization (σ s ),
coercivity (H c ) and effective number of Bohr magneton (η B )
were calculated and found to be σ s = 49 gauss-cc/gm, Hc =61
Oe and η B = 11. High value of saturation magnetization is due
to the presence of Zn ions in tetrahedral sites of both S and T
blocks so that the parallel orientation of the moments in
octahedral sites is favoured. The low value of coercivity
(below 200 Oe) classifies the compound as soft magnetic
material [26]. Because of nonrectangulare shape of loop (low
H c value), hysteresis loss is less and therefore this ferrite may
be used for the low core loss requirements.
Fig; 2 graph of log of DC electrical resistivity versus inverse absolute
temperature
of Ca 2 Zn 2 Fe 12 O 22
(point of inflexion) 408 K. This strongly supports the
influence of magnetic ordering up on the conductivity process
in ferrites. Thus, gave two different values of activation
energy. It is smaller (0.150eV) in ferromagnetic region as
compared above T c , the paramagnetic region (0.28 eV). It is
due to the migration of intrinsic electron between iron ions on
octahedral sites within the spinel blocks [25]. The values
indicate the semiconducting nature of the compound.
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C. Magnetic properties
1) Curie temperature (T c ): The transition point
between
magnetic states was observed by Gouy’s balance [21]. The
ferrimanetic sample was found to be paramagnetic above 400
K. The low value of T c as compared to other Y-ferrite [24]
accounts for the less thermal stability of the compound. The
graph of inverse molar susceptibility (1/χ M ) versus absolute
temperature is shown in Figure 3. It is linear above Curie
temperature showing paramagnetic behaviour. The Curie
molar constant comes out to be 52.45, close to theoretical
value (52.5).
Fig. 3: Variation of inverse molar susceptibility versus absolute temperature
of Ca 2 Zn 2 Fe 12 O 22
Fig. 4: Hysteresis loop for Ca 2 Zn 2 Fe 12 O 22 for magnetic field of 300 Gauss
between poles of electromagnet and CRO set for 50 mV/div.
IV. CONCLUSION
Zn substituted Ca-Y hexagonal ferrite was prepared by
solid state diffusion technique. No deformation in structure is
observed for Ca2+ cations, which replaced the larger barium
ions in Ba-Y ferrite except slight reduction in ‘c’. The
presence of zinc weakens the superexchange interaction which
in effect lowers the Curie temperature. Zinc enhances the
saturation magnetization making it useful in high resonance
frequency applications. Due to non-rectangular trace of
hysteresis the compound may not be useful as permanent
magnet. The sample is a semiconductor having two activation
energies.
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